An experimental and theoretical analysis of the effect of added weight on the energetics and hydrostatic function of the swimbladder of European sea bass (Dicentrarchus labrax)

We conducted experiments to determine the effect of the increasing ultrason ic/radio transmitter weight on the routine metabolic rate of sea bass. We m easured the oxygen consumption (MO2) of fish tagged externally with a dummy transmitter made of a hollow pipe, the weight of which was adjusted with l ead to represent in water 0, 1 and 4% (R-tf) of the animal weight. We then developed a theoretical model to estimate, for a given fish size, the range of added weight that fish can compensate for through swimbladder regulatio n. When R-tf less than or equal to 1%, MO2 of untagged and tagged fish did not differ significantly. However, when R-tf reached 4%, fish that carried a tag incurred a significant elevation of oxygen consumption, which represe nted 28% of their total useable power (or metabolic scope). This result str ongly supports the view that a high R-tf ratio contributes to a decrease in available metabolic energy by diverting energy from, e.g., growth or swimm ing performance. A comparison between the tagged fish and the theoretical m odel reinforced the hypothesis that, when R-tf attained 4%, the increase in metabolic rate reflected a supplementary and costly swimming effort necess ary to maintain vertical position. In this condition, the swimbladder canno t regulate the buoyancy of tagged fish.